1. Field of the Invention
Embodiments of the invention relate to a method for manufacturing a thin film transistor (TFT) substrate, an more particularly to a method for manufacturing a thin film transistor (TFT) substrate using a maskless exposing device.
2. Description of the Related Art
In general, a TFT is used to drive each pixel in a flat panel display (FPD), such as a liquid crystal display (LCD), a plasma display panel (PDP), and an organic light emitting diode (OLED). Hereinafter, the LCD having a TFT substrate will be used as an example to describe the use of a TFT to drive a pixel. The LCD displays images by using electrooptical characteristics of liquid crystal molecules. More particularly, the LCD includes a liquid crystal panel, a backlight unit, and a driving circuit unit. The backlight unit provides light to the liquid crystal panel and the driving circuit unit drives the liquid crystal panel.
The liquid crystal panel displays images using light provided from the backlight unit. The liquid crystal panel includes a TFT substrate and a color filter substrate that face each other with a layer of liquid crystal molecules interposed therebetween. A TFT array, which includes TFTs and pixel electrodes arranged in a matrix for applying voltages across the layer of liquid crystal molecules using a common electrode, is formed on the TFT substrate. A color filter array, which includes red, green, and blue color filters for expressing colors, is formed on the color filter substrate.
The process for fabricating the liquid crystal panel having such a structure will be described hereinafter. First, the TFT array is formed on a first substrate made of either glass or plastic. In addition, the color filter array is formed on a second substrate made of either glass or plastic. Subsequently, a liquid crystal material is dropped onto one of the first and second substrates, and then the first and second substrates are attached. Then the attached first and second substrates are cut, thereby completing fabrication of a liquid crystal panel.
In the process of fabricating the liquid crystal panel, the TFT array on the first substrate and the color filter array on the second substrate are formed through a photolithography process, respectively. In both cases, an exposing process is performed by using an exposing device that uses masks. For example, in order to fabricate the TFT array on the first substrate, five sheets of masks are used and an exposing process is performed per each mask. More specifically, a gate pattern, including gate lines and gate electrodes, and common electrodes are formed through a first masking process. Gate insulation layers, active layers and ohmic contact layers are formed through a second masking process. A data pattern, including data lines and source electrodes, and drain electrodes are formed through a third masking process. A passivation layer is formed through a fourth masking process. A transparent conductive pattern, including pixel electrodes, is formed through a fifth masking process. The large number of masks used in the exposing processes makes the TFT fabrication process complicated and takes a large amount of time to fabricate the TFT, which results in degradation of the productivity of the TFT substrate and the LCD having the TFT substrate.
A technique for fabricating the TFT array in which four masks are used and a single exposing process performed for each of the masks has been developed. More specifically, the gate pattern and the common electrodes are formed through a first masking process. The gate insulation layers, the active layers, the ohmic layers, and the data pattern are formed through a second masking process. The passivation layer is formed through a third masking process. The transparent conductive pattern is formed through a fourth masking process. In the four-mask technique above, the mask employed for the second masking process is a partial exposing mask, such as a slit mask or a half-tone mask.
The masks used for fabricating a device are expensive. In particular, the partial exposing mask is considerably expensive compared to other types of masks. This is because a slit pattern has to be accurately formed in the slit mask at a region where a channel of the TFT is to be formed while the half-tone mask has to be appropriately surface-processed to allow only 50 percent light transmission at particular areas. In addition, because a fabrication of the mask is difficult as the mask becomes larger, the mask fabrication cost increases by geometric progression as the size of the LCD increases in size, and accordingly, the unit cost for manufacturing the TFT substrate and the LCD having the TFT substrate increases.
The current tendency is that large-scale first and second substrates are being employed to enhance the productivity and obtain a large-sized LCD. In this respect, however, the size of masks fails to follow the trend because of the technical limitation of the exposing device. Thus, a dividing and exposing method is employed in which the first and second large substrates are divided into a plurality of exposure regions, which are then exposed through a plurality of shots. Herein, the shot refers to exposing a single exposure region, and a size of each exposure region is determined depending on the size of the shot. This will be described in detail with reference to FIGS. 1 and 2.
FIG. 1 is a plan view for explaining the exposing process through a plurality of shots according to the related art, and FIG. 2 is a plan view showing a stitch deficiency generated between adjacent exposure regions of FIG. 1. With reference to FIGS. 1 and 2, a plurality of shots proceed to form the TFT array 14 on the first substrate 10. When the shots are proceeding, distortion, such as a shift, a rotation, and distortion can occur that create shot misalignment and thus generating discontinuity among adjacent shots, such as adjacent exposure regions 18a, 18b, 18c, and 18d of FIG. 1.
Due to shot misalignment, a luminance difference occurs at the boundary regions of the adjacent exposure regions 18a, 18b, 18c, and 18d. The stitch deficiency refers to a phenomenon in which the boundary regions of the adjacent exposure regions 18a, 18b, 18c, and 18d appear to be like a band to human eyes. Such a stitch deficiency causes degradation of picture quality of the LCD.